1. Field of Invention
The present invention relates to an electronic device, a time difference data acquisition method, and a data structure for time difference data.
2. Description of Related Art
The Global Positioning System (GPS) for determining the position of a GPS receiver uses GPS satellites that circle the Earth on known orbits, and each GPS satellite has an atomic clock on board. Each GPS satellite therefore keeps the time (referred to below as the GPS time or satellite time information) with extremely high precision.
All GPS satellites transmit the same GPS time, and the Coordinated Universal Time (UTC) is acquired by adding the UTC offset (currently +15 seconds) to the GPS time. For an electronic timepiece to receive a satellite signal transmitted from a GPS satellite, acquire the GPS time, and display the local time (regional time) at the location where the electronic timepiece is being used, the time difference to the UTC must be added after correcting for the UTC offset in order to get the current local time, and the electronic timepiece must therefore know what this time difference is.
The UTC offset can be acquired from the data in the received satellite signal, or a predetermined value stored in ROM may be used.
Radio-controlled timepieces and navigation systems that acquire positioning information and time information (UTC) using satellite signals transmitted from GPS satellites, obtain the time difference at the current location from the acquired positioning information, and calculate and display the local time are known from the literature. See, for example, Japanese Unexamined Patent Appl. Pub. JP-A-H08-68848 and Japanese Unexamined Patent Appl. Pub. JP-A-2003-139875.
JP-A-H08-68848 teaches acquiring the time difference information by comparing the positioning data with boundary information data. In order to avoid detecting the wrong time difference information, the device taught in JP-A-H08-68848 must store boundary line information for all time zones in the world in a local storage device.
However, the borders between time zones are often winding national borders, and the amount of data required to store all time zone boundaries is immense. Such boundary line data therefore cannot be stored in small electronic devices such as wristwatches because the available storage capacity is limited by both size and cost constraints. The technology taught in JP-A-H08-68848 is therefore limited in the types of devices in which it can be used, and the technology can more particularly not be used in electronic devices such as wristwatches.
JP-A-2003-139875 teaches extracting fixed position information that is closest to the position of the mobile device to acquire the time difference for that location. More particularly, a circular range is set centered on a fixed position, and if the position of the mobile device is within this range, the time difference for that fixed position is set and used. The possibility of setting the wrong time zone (time difference) is therefore high in areas where the time zone borders are intertwined.
In order to adjust the size of these circular areas, distances are normalized using a weighting coefficient referred to as “fixed range information.” However, when in areas where time zone borders intertwine and there are multiple fixed positions around and near the location of the mobile device, it is difficult to set the fixed ranges so that detection errors do not occur, and the amount of data required to do so increases.
Furthermore, because the distance between the mobile device and each fixed position must be calculated, the calculations are complicated and time-consuming when there are multiple fixed positions in the vicinity of the mobile device, and the technology cannot be used in electronic devices such as wristwatches using low performance processors.